Local area networks are presently utilized, for example, to interconnect such devices as computers, terminals, work stations, printers and telephones within an office environment. For the effective operation of local area networks, the devices are generally configured to passively connect to the transmission medium interconnecting the devices. Due to this passive connection, nearly all failures occurring within the devices do not interfere with signals propagating on the medium and, therefore, failures or outages of a device very rarely affect the correct operation of the remaining system.
Examples of conventional local area network arrangements that utilize passively connected devices include those discussed in U.S. Pat. No. 4,063,220, issued in December, 1977 and U.S. Pat. No. 4,439,763, issued in March, 1984. The first reference describes the arrangement now generally referred to as Ethernet, whereas the second reference describes the so-called Fasnet system. Since Fasnet is representative of conventional systems, it is now discussed in some detail to illustrate the characteristics of local area network arrangements.
Fasnet is typically deployed in office environments requiring efficient system operation at very high transmission speeds. This is achieved by configuring a station at one end of a unidirectional transmission medium to propagate a synchronizing signal or waveform to the remaining stations attached to the medium. According to the protocol described in U.S. Pat. No. 4,439,763, attaching stations are arranged to determine when it is appropriate to transmit coded digital information on the shared medium. The information provided by each station is added to the energy of the synchronizing information already on the transmission medium. A failure of one station generally results in the inability to propagate a coded signal from the inoperative station; the remaining stations operate normally.
One particular code oftentimes utilized in the conventional systems, including Fasnet, is the so-called bipolar code or alternate mark inversion code. Such a code is selected so as to reduce the DC component contained within the overall transmitted waveform. This modulation or coding scheme is appropriate for communication over a medium such as a coaxial cable where it is not difficult to construct a transmitter and receiver pair that linearly transforms the information applied to the medium. However, a bipolar-type signal is a three-level signal. In an optical communication bus realization of a local area network, it is basically very difficult to arrange for the transmitting device, such as a light emitting diode or laser diode, to generate a three-level signal since the source is inherently a two-state, ON-OFF light source.
For this reason, in a point-to-point optical transmission system, it is common in the art to employ a two-level modulation scheme known as Manchester encoding. The digital signal is modulated in such a way that a transition from high-to-low or from low-to-high occurs at the center of every bit period. The frequently occurring bit transitions enable the recovery of a clock signal to aid in the decoding of the digital information conveyed by the waveform propagated on the medium. The Manchester scheme, however, is not appropriate for synchronous multiple access systems because when a station ceases transmitting, the synchronizing information is removed from the medium and the stations attached to the medium lose synchronism. Asynchronous operation is possible, but this is appropriate for only low speed communication. At high speeds, the time required for a station to reestablish synchronization can be a significant fraction of the period of time taken by a station to transmit a packet of information.